Materials in electrophotographic process

ABSTRACT

THE PRESENT INVENTION IS DIRECTED TO A DRY POWDER RESIN FREE TONER COMPOSITION FOR DEVELOPING ELECTROSTATIC IMAGES COMPRISING A MAGNETIC CARRIER AND A FATTY ACID.

March 12, 1974 YOSHIKI HAYASHI ET AL 3,796,664

MATERIALS IN ELECTROPHOTOGRAPHIC PROCESS Original Filed Oct. 4., 1965 FIG. 1

TOP COATED LAYER PHOTO CONDUCTIVE LAYER ADHESIVE LAYER TRANSPARENT CONDUCTIVE LAYER FILM BASE CHARGE INTENSITY (ARB. UNIT) QOI 0.05 0. I 0.5 L0

AMduNT 0F STEARIC ACID m MAGNETITE (WEIGHT PERCENTAGE) United States Patent US. Cl. 252-621 2 Claims ABSTRACT OF THE DISCLOSURE The present invention is directed to a dry powder resin free toner composition for developing electrostatic images comprising a magnetic carrier and a fatty acid.

This application is a division of Ser. No. 492,695, filed Oct. 4, 1965, now Pat. No. 3,438,773.

The present invention is concerned with electrophotographic materials, i.e. materials useful in carrying out electrophotographic processes.

More especially, the present invention is concerned with the constitution of electrophotographic films having a high transmittance and with toner compositions applicable to said films for the purpose of visible presentation of latent electrostatic images.

Electrostatic processes for graphic reproduction and photography have been developed by many investigators for a long period of time, their goals being mainly limited to graphic reproductions on opaque papers. The bestknown system, called Xerography has been disclosed in Journal of the Optical Society of America, vol. 38, No. 12, December 1948. However, transparent photography by an electrostatic process has not been achieved successfully, for the reason that transparent photographs, usually projected in large magnification onto a screen, require higher resolution, less fog and an exact reproduction covering half tone. It is important for obtaining excellent images that the toner consist of finely dispersed pigment powder. Prior toner compositions, usually containing resin binder, have a tendency to aggregate during developing and fixing.

It is an object of the present invention to provide an electrophotographic image with high resolution, less fog and an exact reproduction covering half tone.

It is another object of the invention to provide compositions of toner producing visual images formed of finely dispersed particles.

It is a further object of the invention to provide transparent electrophotographic film constitutions characterized by less fog and high resolution covering half tone.

It is a still further object of the invention to provide a method of fixing toner in excellent images.

These and other objects of this invention will become apparent upon consideration of the following description taken toegther with accompanying drawings in which:

FIG. 1 is a cross-sectional view, on a highly exaggerated scale, of a transparent electrophotographic film in accordance with this invention, and FIG. 2 is a graphical illustration of an effect of additive amount of surface 3,796,664 Patented Mar. 12, 1974 P CC active agent on the charge intensities of toners in accordance with this invention.

Referring to FIG. 1, a transparent conductive layer 2 is coated on a transparent film base 1, and is laminated firmly to an organic photoconductive layer 4 through the medium of an adhesive 3. The organic photoconductive layer 4 is coated with a transparent resin 5 having a tonerfixing capacity which is established upon curing the resin by heating or by treating with vapors of an appropriate organic solvent. It is a feature of this invention that the film is provided with a top coat layer 5 which fixes an imaged toner pigment. According to the prior art, electrophotographic film comprising a top coat layer for fixing is not known; moreover, the toner itself has heretofore contained a resin for fixing imaged pigment. It has heretofore been believed that a top coated layer of non-photoconductive material on the photoconductive material layer would act to lower the electrophotographic light sensitivity and the resolution of image. It has been discovered, according to the present invention, that is suitable nonphotoconductive material of the top coated layer does not lower the sensitivity and/or the resolution.

It is necessary that the top coated layer consist of a resin which has a specific electrical resistivity higher than 10 ohm-cm. and a thickness less than 15,u and that it easily cures into a transparent layer for fixing a toner pigment by means of heating or treating with vapors of a resin-soluble solvent. According to the invention, copolymer of vinyl chloride and vinyl acetate is preferable for the production of the top coated layer. The preferable thickness is of 5 to 10 The transparent conductive layer is required to exhibit 60 to of transmittance in incandescence and a surface resistivity lower than 10 ohm. A thin layer of vacuum-evaporated metal such as copper or chromium can be employed. A thin layer of cuprous iodide in a thickness of 200 to 800 m is most preferable for this purpose and can be prepared as follows: Copper is vacuumevaporated on a sheet of commercial transparent cellulose acetate film and is iodized with iodine vapor in a similar way to that of the prior art; cf. method described in US. Pat. No. 2,756,165, patented July 24, 1956. The thickness of the cuprous iodide layer can be controlled by checking its transmittance. Any defects such as pin holes and scratches must be avoided because they give undesirable defects to resultant images.

An adhesive to laminate the cuprous iodide layer and organic photoconductive layer plays an important role on the quality of image. Polyvinyl acetate in a solvent inert to cuprous iodide and to the base material is preferable for obtaining an excellent resultant image, and is coated on the cuprous iodide layer by a per se well known method, for example, by roller coating, spraying or squeezing in a thickness of 3 to 7a.

The photoconductive layer can be formed of any material having photoconductivity characterized by a dark electric resistivity higher than 10 ohm-cm. It is necessary that the surface of the photoconductive layer be as smooth as that of plate glass. A thin layer of vacuum evaporated inorganic photoconductive material such as selenium, or cadmium sulfide can be employed but is of inferior transparency. Many organic materials having photoconductivity are superior to said inorganic materials from the standpoint of transparency. Organic polymeric materials such as poly-N-vinylcarbazole, polyacenaphthylene, and polyvinyl triphenylpyrazoline can be employed without addition of binders. The following are operable only when accompanied by the addition of binders for formation of thin layers: oxadiazole compounds, leuco malachite-green, tetrahydroimidazole derivatives, pyrazoline derivatives, 4,5-diphenyl imidazolthion derivatives and 1,3,4-triazole derivatives. It is preferable that the photoconductive layer be formed of poly-N-vinylcarbazole having incorporated therein a small amount of Rhodamin 6G and 2-methylanthraguinone in a thickness of 20 The coating method for the poly-N-vinyl-carbazole solution is the same as that of polyvinyl acetate adhesive solution.

In connection with the novel constitution of the electrophotographic film, the toner compositions of this invention are characterized by an elimination of, Le. absence of, resin for fixing the pigment powders. According to this invention, the toner comprise pigment powders and organic compounds which are employed to achieve uniform distribution of the electrostatic charge of the toner pigment. Since a conventional toner contains resin such as coumarone-indene resin, piccopale resin or polystyrol for fixing the pigment powders, these pigment powders have a tendency to aggregate during curing of the resin and result in an inferior image. This disadvantage is removed by using a top coated layer in the film constitution as described above and eliminating the resin in the toner compositions. This is a great feature of this invention.

The toner is brushed onto the top coating layer having a latent electrostatic image by means of a magnetic brush which reveals the image immediately as the brush is swept across the top coated layer. The magnetic brush is per se art-known and consists of a permanent magnet carrying at one end a mass of iron powder loaded with the toner pigment. According to the present invention, it has been discovered that fogs of the visual images are reduced by using a toner pigment comprising fine powders of magnetic material. When the toner contains pigment powders charged with an unfavorable sign or quantity or pigment powders free from electrostatic charge, the magnetic brush picks up these powders and renders the resultant image clean when the magnetic force exceeds the Coulomb force. The operable magnetic materials in a fine powder form are l) magnetic metal such as iron, nickel, cobalt and their alloys, (2) magnetic oxide compound such as magnetite, cobalt oxide (C 0 and ferrites, and (3) magnetic sulfide such as FeS and C08. In producing a toner pigment, it is desirable that the magnetic metal or alloy powder be coated with compound thereof such as oxide, hydroxide, or sulfide for obtaining a high insulating coating by means of per se usual chemical techniques.

It is necessary that the pigment be in a finely divided powder form of less than 10,11. preferably 0.5 to 5 1. particle size. The preferable pigment consists of magnetite in 0.5,u. to 2, particle size in accordance with the present invention.

Of importance is that the amount of electrostatic charge of pigment powder be high and be uniformly distributed throughout the powder and that there exist no positively charged pigment powder during the brushing process. Addition of surface active agents such as stearic acid or palmitic acid are known to be useful for this purpose. According to the invention, preferable surface active agents are fatty acids containing more than carbon atoms in the molecule. The more carbon atoms in the fatty acid, the more desirable the effect. The weight proportion of surface active agent to toner pigment is also required to be controlled. The preferable addition amount of stearic acid or palmitic acid is less than 5%, by weight preferably 0.1 to 1% by weight. An addition of more than 5% by weight of fatty acid results in an unhomogeneous mixture which produces undesirable images.

As described above, the magnetic brush comprises a permanent magnet and a mass of iron powder called a carrier. It is of importance that the carrier be negatively charged and the toner pigment positively charged during brushing. The electrostatic process relating to the toner and the carrier, however, has not yet been definitively clarified. It has been discovered according to the present invention that desirable amounts of negative charge of carrier are satisfactorily achieved by coating the surface of the carrier iron powder with fluorowax. The preferable wax is poly trifiuoro-chloro-ethylene wax having an average molecular weight of 1000 to 1300. Besides the preferable electrostatic properties, excellent lubricating properties between toner and carrier are also produced by applying the said wax in the developing process. From a consideration of fluidity, it is preferred that the carrier iron powder be in the form of spherical particles of 100 to 200p in diameter. A preferable weight proportion of trifluoro-chloro-ethylene polymer wax to the iron powder is 2 to 4% by weight. An application of trifluoromonochloro-ethylene polymer wax to the carrier powder is made in a per se well-known manner, for example, the iron powder is immersed in the molten fluorowax or in a solution thereof in an inert solvent and subsequently dried.

Further, according to the present invention, bonding of the fluorowax to the surface of the carrier iron powder is remarkably enhanced by treating the iron powder with a solution of free phosphoric acid, phosphate compound such as ZnHPO or Na PO and a promoter, prior to the application of the fluorowax. When the iron powder is immersed into a solution of, for example, zinc phosphate, the surface of the iron powder is firmly coated with a thin layer of Zn (PO which is not dissolved by any conventional solvents such as water, alcohol or organic solvent, and is responsible for the strong adhesion of the fluorowax.

The following examples are given to illustrate presently preferred details of the invention, it being understood that the details of the examples are not to be taken as in any way limiting the invention thereto.

EXAMPLE 1 (TONER PREPARATION) Metallic cobalt powder in an average particle size of 1.4;; is heated at 800 C. in air to obtain a partly oxidized black powder. The black powder in an amount of 100 grams is immersed in 20 milliliters of benzene containing 0.5 grams of palmitic acid, and mixed sutficiently in a 'ball mill. The mixture in cream state is dried with hot air at 60 C., and is then evacuated in a vacuum chamber at room temperature (about 20 to about 30 C.) for about five hours, producing a toner free from benzene.

EXAMPLE 2 (TONER PREPARATION) A solution of 20 milliliters of ethanol and various amounts of stearic acid are added to 100 grams of magnetite in an average particle size of 0.2 1. and mixed in a ball mill. The magnetite is obtained by a per se well known chemical technique. The mixture in a cream state is dried with hot air at 60 C. and is then evaculated in a vacuum chamber at room temperature for about five hours, producing a toner free from ethanol.

Referring to FIG. 2 wherein the charge intensity is represented in an arbitrary unit, it will be seen that the charge intensity increases with increase in the additive amount of stearic acid.

EXAMPLE 3 (CARRIER PREPARATION) An aqueous solution of 3% by weight of sodium phosphate is added to l kilogram of electrolyzed iron powder in an average particle size of 100 to 200g. The mixture is heated at C. for 30 minutes under stirring for cleaning the surface of the iron powder. After being washed with water, the cleaned powder is steeped in a hot aqueous solution of 3% by weight of monobasic zinc phosphate and 0.2% by weight of sodium nitrite.

The iron powder in the solution is stirred for 50 minutes in order to form an active phosphate layer on the surface of the iron powder, and then is thoroughly washed with water. In order to increase the durability of the bonding, the treated powder is steeped in an aqueous solution of 0.05% by weight of bichromic acid for 1 minute at room temperature and is quickly dried after being separated from the solution. Poly-trifiuoro-monochloro-ethylene having an average molecular weight of 1500 is added to 100 grams of the iron powder thus obtained. The mixture is heated at 80 C. with stirring. It is desirable for obtaining a homogeneous mixture to add 50 milliliters of methanol, which evaporates ofl completely during the mixing.

EXAMPLE 4 (COMPLETE EXEMPLARY PROCEDURE) Metallic copper is evaporated onto a sheet of commercial cellulose diacetate film of 150 in thickness. The white light transmittance of the evaporated film is 63%. The copper evaporated film is placed immediately in a glass chamber containing a small amount of crystalline iodine powder. As the air of the chamber is evacuated, there is a diminishing of the copper color, and finally the film becomes transparent. The transmittance of the cuprous iodide laminated film is 90% with white light and the surface resistance is ohm-cm. A toluene solution of by weight of polyvinyl acetate is coated onto said cuprous iodide film by a Baker type applicator and is dried thoroughly. The dried lacquering layer is 4 in thickness.

Coating of both the photoconductive layer and the top layer is carried out in a manner similar to that of the polyvinyl acetate adhesive layer. The compositions of both solutions are given in Tables 1 and 2, respectively. The thickness of the two dried layers are 18 and 10 respectively.

TABLE 1.-COMPOSITIONS OF PHOTOSENSITIVE LAYER Poly-N-vlnylcarbazole is commercially available e.g. as Luvicun M170; BASE.

*See Schultz Farbstofftabellen, 7th edition, 1st vol., 1931, p. 366, No. 866.

6 TABLE 2.CO'MPOSITIONS OF TOP LAYER Material: Parts by weight Polyvinylchloride acetate 1 (top coating matrix) Epoxy resin (plasticizer) 25 Chlorinated paraffin (plasticizer) 15 Methyl isobutyl ketone (solvent) Toluene (solvent) 350 1 CCommercially available e.g. under the trade name VAGH;

Commercially available e.g. as Epikote 828; Shell.

The pale orange transparent electrophotographic film thus obtained is mounted on a 7 kv. corona generation apparatus and is charged up to 1000 negative volts in the dark, after which a static latent image of a subject to be reproduced is created by the conventional exposure method. The optimum exposure is 500 lux-sec.

The developer consisting of one part by weight of toner and ten parts by weight of carrier, obtained by the process according to Example 2 and Example 3 respectively, is mixed thoroughly with a permanent magnetic stick and a magnetic brush is formed. By sweeping the brush onto said latent image, an excellent visual image i developed.

Finally the visual imaged film is placed in a glass chamber containing dense vapors of carbon tetrachloride for a period of time less than 2 seconds. The image is fixed stably. The resolution of the resultant fixed image is 50 lines per mm., and its reproduction of half tone is 7 steps by the Eastman Kodak No. 2 grey scale; and no fog is observed.

What is claimed is:

1. A dry powdered resin-free toner for electrophotography which is capable of being developed by a magnetic brush development and consists essentially of 0.5 weight parts of palmitic acid and 100 weight parts of partly oxidized cobalt powder with an average particle size of about 1.4,u.

2. A dry powdered resin-free toner for electrophotography which is capable of being developed by a magnetic brush development and consists essentially of 0.1 to 1.0 weight percent of stearic acid and 99.9 to 99.0 weight percent of magnetite powder with an average particle size of about 0.2a.

References Cited UNITED STATES PATENTS I. TRAVIS BROWN, Primary Examiner J. P. BRAMMER, Assistant Examiner 

